CoGHiLL, T he Reaction to Tactile Stimuli. 97 



is regularly away, from the side touched when the stimulation is 

 applied to the fields of the n. trigeminus and n. vagus. 



B. Response to Siunulatlon of the Tail Bud. 



There is no marked regularity in the responses to touches on the 

 tail bud. There is a slight general tendency in some specimens 

 towards movemeHt of the head toward the side touched, but no definite 

 significance can yet be attributed to this tendency. It is clear, how- 

 ever, that specimens that are asymmetrical with reference to stimula- 

 tion on the head are similarly asymmetrical with reference to stim- 

 ulation of the tail bud, and that ordinarily the asymmetry with 

 reference to the two points of stimulation extends over approximately 

 the same period. 



One other fact concerning the reaction to stimulation on the tail 

 bud is established beyond question by my experiments. The first 

 response to such a stimulus in the very young embryo is a head move- 

 ment, and as the embryo advances in age this movement still begins 

 in the head region and progresses caudad. Ontogenetically, then, 

 the most primitive conduction paths of the medulla spinalis are 

 longitudinal and afterent, and the crossed paths are secondary, ex- 

 cepting possibly in the most cephalic part where the medulla spinalis 

 may be involved in the crossed paths between the n. trigeminus or 

 n. vagus and the opposite musculature of the trunk. The two halves 

 of the medulla spinalis, therefore, seem to be physiologically distinct 

 during this phase of development. This fact of development reveals 

 from a new source the fundamental nature of the longitudinal divi- 

 sions of the cerebro-spinal system, at least of the somatic components, 

 as they have been conceived by Herrick,^ Johnston* and others on 

 purely morphological and physiological grounds. It also suggests 

 that in their direct connection with the cephalic part of the nervous 

 system the special cutaneous systems of fishes and amphibians accord 

 essentially with the ]irimary plan of the general cutaneous system. 



^"Tlie Cranial and First Spinal Nerves of Menidia," Arc-hives of Neurology 

 and Psychology, Vol. II, and The .Tournal of Comparative Neurology, Vol. 

 IX ; also numerous later papers, mostly in this Journal. 



'"The Brain of Acipenser," Zool. Jahrh., 1901 ; "The Nervous System of 

 Vertebrates," Philadelphia, 190G ; and other papers in this Journal. 



